Group 8
Eighteen second lactation Holstein cows averaging 89.2 (± 8.1) DIM were randomly assigned to an environmental chamber room into individual tie stalls located at the University of Arizona William J. Parker Research Complex. The chamber rooms only house six cows at a time therefore the study was replicated three times and one cow was removed from the study due to temperament issues in the facilities. All cows were milked two times a day and milk weights were recorded at each milking (0500, 1700 h). All cows were fed a totally mixed ration (TMR) two times day at milking times and orts were recorded prior to the morning feeding. Cows were housed at the University of Arizona dairy for 19 days prior to entering the environmental chambers. While at the dairy, cows received one of two dietary treatments 1) Control diet with 0 g/ton Rumensin or 2) the control diet top dressed with Rumensin at 450 mg/cow/d. Once they entered the facility they were provided 3 d to adjust however, continued on their dietary treatment. All cows regardless of dietary treatment experienced a constant thermal neutral environment (20% humidity, THI = 64, with 14 h light and 10 h dark cycles) and allowed to eat ad libitum for 9 d (experimental period [P] 1). They were then given 2 d in the same thermal neutral environment prior to experiencing P2 (experimental period [P] 2) which consisted of cyclical temperatures (29.4 to 38.9°C with constant 20% humidity, THI ≥73, ≤ 82 and 14 h light and 10 h dark cycles) and were fed ad libitum. This environment was made to replicate daily